FAA Aviation Mechanic Powerplant (AMP)Turbine EnginesMedium
A turboprop engine's power (free) turbine rotates at approximately 30,000 RPM, but the propeller must turn at a much lower, aerodynamically efficient speed. Which component allows the propeller to operate at this reduced rotational speed?
- ACompressor bleed valve
- BReduction gearbox
- CFree turbine assembly
- DAccessory drive gearbox
Show answer & explanationAnswer & explanation
Correct answer: B. Reduction gearbox
The reduction gearbox uses a gear train to step down the high rotational speed of the power turbine shaft to a much slower speed suitable for efficient propeller operation, typically reducing speed by a ratio of roughly 10:1 or more depending on the engine model.
Why the other options are wrong
- A. Bleed valves manage compressor airflow and have no relation to propeller speed reduction.
- C. The free turbine drives the reduction gearbox but does not itself reduce speed for the propeller.
- D. The accessory gearbox drives engine accessories (pumps, generators), not the propeller.
Turboprop Reduction Gearbox
A gear train that reduces the high rotational speed of a turboprop's power turbine shaft to a lower speed suitable for efficient propeller operation.
- Typical reduction ratios can exceed 10:1
- Connects free/power turbine to propeller shaft
- Essential because propellers are inefficient at high RPM
Memory trick: 'Gearbox Grinds Gears down' from fast turbine to slow prop.